Answer: Oxymoron
An oxymoron is a rhetorical device that represents a contradiction or reveals a paradox. However, the oxymoron is a deliberate contradiction, as opposed to one that arises from confusion or by mistake. Oxymorons are extremely common in literature. In this case, the author uses an oxymoron by saying "heavy lightness" as lightness, by definition, is "light," which is the complete opposite of "heavy."
